In telecommunication networks, digital signals undergo signal distortion due to dispersion and other physical effects on their way through the network. At the receiver side, the received digital signal must be regenerated to recover the original signal. This regeneration includes a decision step wherein the received “analogue” signal, after O/E conversion, pre-filtering and/or dispersion compensation where necessary, is compared to a decision threshold at regular instants in time, which correspond to the data clock of the received signal, to decide the respective bit values of the corresponding signal bits.
This comparison requires an accurately adjusted decision threshold. The more noise the received signal carries, the more accurate must the decision point be adjusted to achieve correct decisions with sufficiently low bit error rate. Decision threshold and decision phase are, however, heavily signal dependent. These parameters must be adjusted automatically so as to obtain “good” decisions for all relevant signal waveforms.
The article “Decision-point steering in optical fibre communication systems: theory” by M. Sherif et al., IEE Proceedings, Vol. 136, Pt. J, No. 3 June 1989, describes a regenerator with a pseudo error monitor, which has a shifted threshold and which serves to produce a pseudo error signal that steers the decision threshold of a decision gate. Using the pseudo error signal as a feedback signal, the decision threshold is dynamically optimized.
This optimization requires “reasonable” starting values, particularly for optical signals that have, undergone massive signal dispersion. Fixed starting values can be used but which is not very flexible and is applicable only for restricted set of waveforms. Alternatively, it would be possible to use information on the signal quality, i.e., the bit error rate, from other, external sources to initially control the threshold and/or phase using this information. An external source may for example be the Forward Error Correcting code (FEC) or other higher layer protocols. Depending on the system architecture, however, such external sources are not always available and involve extra costs due to required interfaces to these devices.
EP 0 455 910 A2 describes a method for distortion compensation by adaptively setting the decision threshold based on at least one prior decision. In other words, prior decisions are used as feedback signal to adjust the decision threshold for subsequent bits. This is similar to the functional principle of a decision feedback equalizer.
EP 0 923 204 B1 uses a pseudo error monitor to control parameters of a pre-filter in an optical receiver.
The problem to be solved by the present invention is hence to provide a method and related apparatus for determining “good” starting values for a decision threshold and/or decision phase without requiring information on signal offset, noise and/or other signal distortion in advance.